Bug Description

Steps to reproduce the problem:
1. Go to the Chromium snap store page by e.g. right clicking on its icon and selecting Show details and then the specific app.
2. Press the Permissions button.
3. Press/slide the Read/write files on removable storage devices slider.
4. Close the permissions and the store window.
5. Reopen the windows.
6. The permission should be enabled, but it isn't.

I can observe the problem in an up-to-date cosmic VM. The interface is correctly connected, but gnome-software fails to display the switch in the correct state. Killing gnome-software "fixes" the problem. So it looks like the status of the permissions are cached in the running instance of gnome-software, and not properly updated when changing them from the UI.